1. Field of the Disclosure
This invention relates to systems for the production of well fluids, including for example oil and gas, from boreholes, and to production tubing and electric submersible pump assemblies for deployment in boreholes.
2. Description of the Related Art
An electric submersible pump assembly (hereafter referred to as an ESP) is deployed in oil wells and other boreholes to transport fluid to the surface, and comprises a pump, i.e. an impeller or other element that acts on the well fluid, coupled to an electric motor that drives it. (It will be understood by those skilled in the art that “a pump” and “an electric motor” include a stack of pumps or a stack of electric motors acting together so as to increase the power of the ESP.)
Production tubing may be either sectional, jointed tubing or continuous, coiled tubing, which is lowered down the borehole to provide a conduit through which the well fluid may be pumped to the surface. With the production tubing in place in the borehole, the ESP may then be lowered down the production tube on a flexible tether to a deployed position, typically proximate its lower end, and then sealed to the internal wall of the tubing by a packer so that the outlet of the pump is in fluid communication with the upper portion of the tube, which is used to conduct the well fluid to the surface. Conveniently, the flexible tether may incorporate an electric cable for supplying power to the motor. Alternatively, the tether may comprise a coiled tube, which may be used to conduct the well fluid to the surface, in which case the ESP may simply be suspended in the production tubing without a seal.
An arrangement of this general type is disclosed for example in US 2007/0289747 A1.
The motor of the ESP generates heat in service, and depending on the power of the pump, may require cooling to ensure the insulation and lubricants of the motor do not break down through excessive heat and damage the motor.
At low power, the static, ambient well fluid may be used to dissipate heat from the motor. However, as the power of the motor (or the temperature of the ambient fluid) increases, the static well fluid is no longer capable of cooling the motor and alternative methods have to be used. One known solution involves placing a shroud around the motor and passing fluid through this shroud. This cools the motor more than the ambient well fluid alone would, but at the expense of more components, greater cost and increased diameter of the pump assembly.
Alternatively, the motor may be cooled by allowing the well fluid passing through the pump to flow over the surface of the motor within the production tube.
In order to provide a conduit between the outer wall of the motor and the inner surface of the production tube, sufficient to carry the full flow of the well fluid passing through the pump so that the well fluid may cool the motor, the motor must necessarily be of substantially smaller diameter than the inner diameter of the production tube. This in turn disadvantageously limits the power of the motor and hence the output of the ESP.
Rather than reducing the diameter of the motor, the diameter of the production tube may be increased, which however substantially increases its cost. Moreover, the larger diameter of the production tube reduces the velocity of the well fluid, which in turn reduces its capacity to carry particulates from the well, leading to a buildup of sand and other debris which can clog the pump and the wellbore.
In practice, it is found in that, even where the motor is cooled by the well fluid passing over its surface within the production tube, overheating may still occur.